Method and device for analyzing a sensor element

ABSTRACT

A method and a device for analyzing a sensor element are described, which allow a temperature dependence of the sensor element to be considered without an additional temperature sensor in particular. The sensor element outputs a signal as a function of a measured variable. The signal of the sensor element is analyzed in a first analysis operating mode to detect the measured variable. The signal of the sensor element is analyzed in a second analysis operating mode to detect a characteristic variable of the sensor element which is different from the measured variable.

FIELD OF THE INVENTION

The present invention is directed to a method and a device for analyzinga sensor element.

BACKGROUND INFORMATION

Methods and devices for analyzing a sensor element are available, thesensor element outputting a signal as a function of a measured variable.

It has been suggested that for controlling internal combustion engines,the combustion chamber pressure is determined, in addition to othermeasured variables. Multiple publications describe technical approachesfor detecting the combustion chamber pressure.

Measuring sensors or sensor elements which operate according to thepiezoelectric principle appear particularly attractive. A suitablematerial, such as quartz or a sintered ceramic material, is subjected tothe combustion chamber pressure. The material is, for example, installedas a disk in a suitable housing and mounted in a conventional way in acylinder head as a combustion chamber pressure sensor. Furthermore,integration into an already existing component, such as a spark plug orglow plug, is conventional. A charge, which is proportional to thepressure, arises in the material of the sensor element subjected to thecombustion chamber pressure, which may be converted into a voltagesignal using a suitable electronic circuit, e.g., a charge amplifier orimpedance transformer. This voltage is processed further in an enginecontrol unit and incorporated into different closed-loop and open-loopcontrols of the engine. The combustion chamber pressure of each cylinderof the internal combustion engine is typically sampled synchronouslywith a crankshaft angle, for example, at a resolution of 1° crankshaftangle.

If a piezoceramic material, such as the sintered ceramic material, isused for the measuring sensor or the sensor element, it is distinguishedby a comparatively high sensitivity, i.e., it generates a greater chargeat a given pressure than quartz. A disadvantage in this case is thepronounced temperature dependence of the sensitivity in suchpiezoceramic materials. Therefore, determining the temperature of thesensor element would be advantageous, in order to be able to suitablycompensate for the temperature error in subsequent signal processing.

Conventional approaches include mounting a temperature sensor in theproximity of the sensor element to detect the combustion chamberpressure, also referred to in the following as a combustion chamberpressure sensor. Alternatively, the temperature error may also becompensated for using a capacitive charge divider, the correspondingcapacitor of the capacitive charge divider having to have the sametemperature as the combustion chamber pressure sensor. This is describedin “A. Peterson, Temperaturkompensation piezokeramischer Sensoren[Temperature Compensation of Piezoceramic Sensors], Elektronikindustrie12-1988.” Both of these methods have the disadvantage of requiring thatstill further components be housed in the already restrictedinstallation space of the combustion chamber pressure sensor.

SUMMARY

A method and device according to an example embodiment of the presentinvention for analyzing a sensor element may have the advantage over therelated art that the signal of the sensor element is analyzed in a firstanalysis operating mode to detect the measured variable, and the signalof the sensor element is analyzed in a second analysis operating mode todetect a characteristic variable of the sensor element which isdifferent from the measured variable. In this way, both the measuredvariable and also the characteristic variable of the sensor elementwhich is different from the measured variable may be derived from thesignal of the sensor element. A separate sensor for detecting thecharacteristic variable of the sensor element which is different fromthe measured variable is therefore not necessary, nor is theconventional compensator circuit. The functionality of the sensorelement signal is thus enhanced.

It may be particularly advantageous if the changeover between the twoanalysis operating modes is performed as a function of at least onecontrolled variable. In this way, it is ensured that the signal of thesensor element is either analyzed in the first analysis operating modeor in the second analysis operating mode, but not simultaneously in bothanalysis operating modes. Therefore, the analysis operating modes mayadditionally be defined as a function of the at least one controlledvariable and thus set, for example, under suitable operating conditionsin each case.

It may be particularly advantageous if a combustion chamber pressuresensor for detecting a combustion chamber pressure of an internalcombustion engine is selected as the sensor element. In this way, thecombustion chamber pressure may be detected from the signal of thecombustion chamber pressure sensor in the first analysis operating modeand the temperature of the combustion chamber pressure sensor may bedetected in the second analysis operating mode, so that neither anadditional temperature sensor in the proximity of the pressure sensornor the compensation for the temperature influence using a capacitivecharge divider is necessary.

If the sensor element is implemented as a combustion chamber pressuresensor of an internal combustion engine, a crankshaft angle of theinternal combustion engine may be selected as the controlled variable,through which different operating states of the internal combustionengine which are a function of the crankshaft angle may be assigned toone of the two analysis operating modes in a particularly simple andreliable manner in each case, so that the particular analysis operatingmode may also be activated as a function of the occurrence of theassigned operating state of the internal combustion engine using thecrankshaft angle.

A further advantage may result if the second analysis operating mode fordetecting the characteristic variable of the sensor element which isdifferent from the measured variable is set for a cylinder of theinternal combustion engine during at least one exhaust stroke of thiscylinder. During the exhaust stroke, determining the combustion chamberpressure and/or determining features of the combustion in general arenot significant, so that this operating phase of the cylinder may beused to detect the characteristic variable of the sensor element whichis different from the measured variable without impairing the analysisof the measured variable. Furthermore, it may be advantageous if afrequency of the setting of the second analysis operating mode fordetecting the characteristic variable of the sensor element which isdifferent from the measured variable is selected as a function of a rateof change of the characteristic variable. In this way, the setting ofthe second analysis operating mode may be reduced to a minimum. Thelower the rate of change of the characteristic variable, the lessfrequently must the detection of the characteristic variable berefreshed and/or repeated to update the values for the characteristicvariable.

It may be advantageous in connection with implementation of the sensorelement as a combustion chamber pressure sensor in particular if atemperature or a capacitance is analyzed as the characteristic variableof the sensor element.

In this case, to detect the temperature or capacitance, the sensorelement may be incorporated in a particularly easy and less complex wayinto an oscillator circuit, in particular an astable multivibrator,which generates a frequency which is a function of only the capacitanceof the sensor element. In this way, the capacitance of the sensorelement may also be determined in a particularly simple and reliablemanner.

Furthermore, it may be advantageous if, to detect the measured variable,the sensor element is incorporated into an impedance transformer circuitor into a charge amplifier circuit. Because of the similarities incircuitry between the charge amplifier circuit or the impedancetransformer circuit and the oscillator circuit, both the first analysisoperating mode and the second analysis operating mode may thus beimplemented with minimum circuitry outlay.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention are illustrated in thefigures and described in greater detail below.

FIG. 1 a) shows a circuit for an astable multivibrator.

FIG. 1 b) shows a curve of different voltages of the astablemultivibrator plotted against time.

FIG. 2 shows a charge amplifier.

FIG. 3 shows an impedance transformer.

FIG. 4 shows an example circuit according to the present invention whichincludes both a charge amplifier and an astable multivibrator.

FIG. 5 shows a curve of a combustion chamber pressure plotted against acrankshaft angle.

FIG. 6 shows a schematic view of an internal combustion engine.

FIG. 7 shows a flow chart for an exemplary sequence of the methodaccording to the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In FIG. 6, reference numeral 5 identifies an internal combustion enginewhich drives a vehicle, for example. In this case, internal combustionengine 5 may be implemented as a gasoline engine or as a diesel engine,for example. In the following, it is assumed as an example that internalcombustion engine 5 is implemented as a gasoline engine.

Gasoline engine 5 includes one or more cylinders. One of these cylindersis shown as an example and identified by reference numeral 10 in FIG. 6.

Fresh air may be supplied to a combustion chamber 35 of cylinder 10 viaan air supply 45 and an intake valve 50. Intake valve 50 may beactivated in this case by a controller 75, for example. Fuel is supplieddirectly to combustion chamber 35 via a fuel injector 65. Fuel injector65 is also activated by controller 75 as needed to achieve a desiredinjected fuel quantity and a desired start of fuel injection. Theair/fuel mixture produced in combustion chamber 35 in this way isignited using a spark plug 70, which is also activated by controller 75to set a suitable moment of ignition, for example. A piston 40 of thecylinder 10, which drives a crankshaft (not shown in FIG. 6), is movedby the energy released during the combustion of the air/fuel mixture incombustion chamber 35. The exhaust gas produced during the combustion ofthe air/fuel mixture is exhausted via an exhaust valve 60 into anexhaust system 55, exhaust valve 60 also being activated by controller75 as shown in FIG. 6, for example. Furthermore, controller 75 may alsoset a desired air supply by activating a final control element not shownin FIG. 6, such as a throttle valve in air supply 45. The fuel injectionmay also occur downstream from the throttle valve or upstream from thethrottle valve into air supply 45, instead of directly into combustionchamber 35, as shown in FIG. 6.

Furthermore, a sensor element 1 is provided, which is implemented inthis example as a combustion chamber pressure sensor. Combustion chamberpressure sensor 1 may be mounted in the cylinder head or in combustionchamber 35 of cylinder 10, in a conventional manner. Alternatively, thecombustion chamber pressure sensor may also be integrated into analready existing component. In the example shown in FIG. 6, combustionchamber pressure sensor 1 is positioned on spark plug 70 and/orintegrated therein, in the combustion chamber side area of spark plug70. In the case of a diesel engine, combustion chamber pressure sensor 1may be positioned on a glow plug in the combustion chamber of the dieselengine or integrated into this glow plug, for example. Alternatively,combustion chamber pressure sensor 1 may also be positioned outside thecombustion chamber in a way known to those skilled in the art.Combustion chamber pressure sensor 1 produces a signal as a function ofthe combustion chamber pressure in the way described above, in the formof a charge proportional to the combustion chamber pressure, forexample. For this purpose, combustion chamber pressure sensor 1 may beimplemented according to the piezoelectric principle as described above,for example, and have a quartz or a sintered ceramic material installedin a suitable housing in the form of a disk, for example. In thefollowing, it is to be assumed for exemplary purposes that combustionchamber pressure sensor 1 is implemented according to the piezoelectricprinciple and includes a disk made of sintered piezoceramic material ina suitable housing.

A charge proportional to the combustion chamber pressure arises in thepiezoceramic material of combustion chamber pressure sensor 1, which isalso referred to as the signal of combustion chamber pressure sensor 1in the following. This charge, i.e., this signal, may be converted intoa voltage signal using a suitable electronic circuit, e.g., a chargeamplifier as shown in FIG. 2 or an impedance transformer as shown inFIG. 3. This voltage is processed further in controller 75 and may beincorporated into different closed-loop and open-loop controls ofinternal combustion engine 5. In this way, the signal of combustionchamber pressure sensor 1 is analyzed to detect the combustion chamberpressure as the measured variable by converting the charge of thepiezoceramic material of combustion chamber pressure sensor 1, which isproportional to the combustion chamber pressure, into the voltagesignal.

A circuit for a charge amplifier, which may be integrated with theexception of combustion chamber pressure sensor 1 into controller 75, isshown for exemplary purposes in FIG. 2. Alternatively, charge amplifier25 shown in FIG. 2 may also be housed in the proximity of combustionchamber pressure sensor 1, e.g., in the same housing. Charge amplifier25 shown in FIG. 2 includes an operational amplifier OP, whosenon-inverting input is connected to a reference potential, to ground inthe example shown in FIG. 2. The inverting input of operationalamplifier OP is connected to a terminal of combustion chamber pressuresensor 1, whose other terminal is also connected to the referencepotential. Furthermore, the inverting input of operational amplifier OPis connected via a parallel circuit having a first capacitor C₀ and afirst ohmic resistor R₀ to an output of operational amplifier OP. Firstcapacitor C₀ and first resistor R₀ are in the negative feedback ofcharge amplifier 25. The potential difference between the output ofoperational amplifier OP and the reference potential is then the voltagesignal converted from the charge of the piezoceramic material ofcombustion chamber pressure sensor 1 and is referred to in the followingas signal voltage u_(M).

Alternatively, signal voltage u_(M) may be converted from the charge ofthe piezoceramic material of combustion chamber pressure sensor 1 usingthe impedance transformer. FIG. 3 shows an example of such an impedancetransformer, which may be integrated with the exception of combustionchamber pressure sensor 1 into controller 75, like the charge amplifiershown in FIG. 2, or alternately may be positioned in the proximity ofcombustion chamber pressure sensor 1, for example, in the same housingas combustion chamber pressure sensor 1. Impedance transformer 20 shownin the example in FIG. 3 in turn includes operational amplifier OP,whose non-inverting input is connected to a terminal of combustionchamber pressure sensor 1, and whose other terminal is connected to thereference potential, in the present example to ground. The invertinginput of operational amplifier OP is connected to the output ofoperational amplifier OP. Signal voltage u_(M) is then applied betweenthe output of operational amplifier OP and the reference potential.Impedance transformer 20 shown in FIG. 3 may, for example, haveamplifier 1 as a special case of the electrometer amplifier.

Sensor voltage u_(S) produced by the charge of the piezoceramic materialat combustion chamber pressure sensor 1 may be amplified and/orconverted as the sensor signal into signal voltage u_(M) at the outputof operational amplifier OP both by the charge amplifier shown in FIG. 2and also by the impedance transformer shown in FIG. 3.

As described above, the cited piezoceramic materials are characterizedby a comparatively high sensitivity, i.e., they generate a greatercharge at a given combustion chamber pressure than in the case wherequartz is used. A disadvantage in this case is, as described, thepronounced temperature dependence of the sensitivity in the case of apiezoceramic material.

According to an example embodiment of the present invention, a secondanalysis operating mode is provided in addition to the described firstanalysis operating mode for detecting signal voltage u_(M), in which acharacteristic variable of combustion chamber pressure sensor 1 which isdifferent from the measured variable, i.e., in this example from thecombustion chamber pressure or signal voltage u_(M), is analyzed. Thischaracteristic variable of combustion chamber pressure sensor 1 may, forexample, be the temperature or the capacitance of combustion chamberpressure sensor 1. There is a connection between the capacitance and thetemperature of combustion chamber pressure sensor 1 which may be storedin a characteristic curve calibrated on a test bench, for example.Therefore, the temperature of combustion chamber pressure sensor 1 maybe inferred from the capacitance of combustion chamber pressure sensor1. A characteristic curve calibrated on a test bench may also, forexample, represent the relationship between the sensitivity ofcombustion chamber pressure sensor 1 and the temperature of combustionchamber pressure sensor 1. Signal voltage u_(M) may then be corrected asthe characteristic variable for the combustion chamber pressure as afunction of the current sensitivity of combustion chamber pressuresensor 1. This correction may therefore be performed if the capacitanceof combustion chamber pressure sensor 1 is known and with the aid of thetwo characteristic curves described. FIG. 1 a) shows a circuit systemhaving operational amplifier OP, with the aid of which a capacitance maybe determined indirectly through a frequency measurement. The circuitsystem shown in FIG. 1 a) is an oscillator circuit which produces asquare wave, for example, whose frequency, at fixed values of the ohmicresistors of the circuit system, is only a function of capacitor C_(S)of combustion chamber pressure sensor 1. This oscillator circuit may,for example, be an astable multivibrator. This is identified in FIG. 1a) by reference numeral 15. Combustion chamber pressure sensor 1 isconsidered as capacitor C_(S) of combustion chamber pressure sensor 1having a temperature-dependent value for this purpose and incorporatedinto the oscillator circuit of astable multivibrator 15. Such an astablemultivibrator is described in, for example, “Horst Wupper,Professionelle Schaltungstechnik mit Operationsverstärkern [ProfessionalCircuit Engineering using Operational Amplifiers], Franzis-Verlag,1994.” As shown in FIG. 1 a), astable multivibrator 15 in turn includesoperational amplifier OP, whose inverting input is connected to aterminal of capacitor C_(S) of combustion chamber pressure sensor 1. Theother terminal of capacitor C_(S) of combustion chamber pressure sensor1 is connected to a reference potential, in the present example toground. First ohmic resistor R₀ connects the inverting input to theoutput of operational amplifier OP. The non-inverting input ofoperational amplifier OP is connected via a second ohmic resistor R₁ tothe output of operational amplifier OP and via a third ohmic resistor R₂to the reference potential. The voltage between the output ofoperational amplifier OP and the reference potential is identified inFIG. 1 a) by u₀. The voltage from the inverting input of operationalamplifier OP to the reference potential is identified in FIG. 1 by u_(C)and corresponds to sensor voltage u_(S) shown in FIG. 2 and FIG. 3.

FIG. 1 b) shows a diagram for the curve of a voltage u(t) over time t.In this case, voltage u₀ and, in addition, voltage u_(C) are illustratedin the diagram shown in FIG. 1 b). Voltage u₀ executes a square waveoscillation having the period T between value U₀₁ and value −U₀₁.Voltage U_(C) also oscillates with the same period, the capacitor beingcharged with capacitance C_(S) when output voltage U_(O) of operationalamplifier OP assumes value U₀₁ and otherwise being discharged. Thefrequency of the square wave of output voltage u₀ may be calculated frommeasured period T through inverse value calculation. The period may bedetermined with the aid of a counter in a microprocessor of controller75, for example. Capacitance C_(S) of combustion chamber pressure sensor1 may be determined in a conventional way from frequency and/or period Tand fixedly predefined ohmic resistances R₀, R₁, and R₂. Therefore, thefrequency of square wave u₀ of the astable multivibrator with fixedlypredefined ohmic resistances R₀, R₁, and R₂ is only a function ofcapacitance C_(S) of combustion chamber pressure sensor 1. If the signalof combustion chamber pressure sensor 1 is analyzed in the firstanalysis operating mode to detect signal voltage u_(M) and in the secondanalysis operating mode to detect capacitance C_(S) of combustionchamber pressure sensor 1, this may be performed by analyzing the signalof combustion chamber pressure sensor 1, i.e., sensor voltage u_(S),with the aid of the charge amplifier shown in FIG. 2 and/or theimpedance transformer shown in FIG. 3 to detect signal voltage u_(M).For the second analysis operating mode, the signal of combustion chamberpressure sensor 1, i.e., voltage u_(C) at capacitance C_(S) ofcombustion chamber pressure sensor 1, which corresponds to sensorvoltage u_(S), may be analyzed using the astable multivibrator shown inFIG. 1 a) to determine capacitance C_(S) of combustion chamber pressuresensor 1. The changeover between the two analysis operating modes may beperformed as a function of a controlled variable, for example. Forexample, crankshaft angle KW of internal combustion engine 5 may beselected as the controlled variable. This means that the first analysisoperating mode is executed for a first range of crankshaft angle KW andthe second analysis operating mode is executed for a second range ofcrankshaft angle KW.

In this case, the second analysis operating mode for detectingcapacitance C_(S) of combustion chamber pressure sensor 1 mayadvantageously be executed during at least one exhaust stroke ofcylinder 10. In the phase of the exhaust stroke of cylinder 10,determining features of the combustion and, in particular, detecting thecombustion chamber pressure is not significant or of interest, so thatin this phase the second analysis operating mode for detectingcapacitance C_(S) of combustion chamber pressure sensor 1 may beexecuted without impairing the analysis of the combustion chamberpressure. The changeover into the second analysis operating mode andtherefore into the oscillator operation according to the circuit systemshown in FIG. 1 a) may be performed in every operating cycle of thecylinder and, therein, in the exhaust stroke of the cylinder.Alternatively, the changeover into the second analysis operating modemay also be performed only in every nth cycle and, therein, in theexhaust stroke of cylinder 10. For this purpose, the thermal behavior ofpiezoelectric combustion chamber pressure sensor 1 may be examined and asuitable frequency for changing over to the second analysis operatingmode may be determined and preferably programmed in controller 75 as afunction of its time constant, i.e., of the rate of change ofcapacitance C_(S) of combustion chamber pressure sensor 1 over time. Thelower the rate of change of capacitance C_(S) of combustion chamberpressure sensor 1 over time, the less frequently is a changeoverrequired into the second analysis operating mode to detect capacitanceC_(S) of combustion chamber pressure sensor 1. The rate of change ofcapacitance C_(S) of the combustion chamber pressure sensor may also bea function of the operating point of internal combustion engine 5, forexample. The changeover frequency, i.e., number n, may thus also beselected as a function of the operating point of internal combustionengine 5. In this case, for example, an associated rate of change ofcapacitance C_(S) of combustion chamber pressure sensor 1 may bedetermined over time for each different operating point of the engineand stored in a characteristic curve on the test bench, the individualrates of change in turn each being assigned a changeover frequency forchanging over to the second analysis operating mode in a furthercharacteristic curve, which may also be calibrated suitably on a testbench. Therefore, with the aid of these two characteristic curves, therequired changeover frequency for changing over to the second analysisoperating mode may be inferred from the operating point of internalcombustion engine 5. In general, the changeover frequency may be setlower the lower the rate of change of capacitance C_(S) of combustionchamber pressure sensor 1.

In a particularly simple and inexpensive way, the changeover between thetwo analysis operating modes may be implemented by designing one singlecircuit for executing the two analysis operating modes. Such a circuitis illustrated in FIG. 4. This circuit system is identified by referencenumeral 30 and represents an expansion of the already existing circuitfor analyzing sensor signal u_(S) and converting it into signal voltageu_(M), which is processed further in controller 75. Circuit 30 shown inFIG. 4 builds on the charge amplifier shown in FIG. 2. In this case, theinverting input of operational amplifier OP is connected via acombustion chamber pressure sensor 1 to the reference potential, in thisexample to ground. The voltage, which drops across combustion chamberpressure sensor 1, is also identified in FIG. 4 by u_(S) as the sensorvoltage. The inverting input of operational amplifier OP is connectedvia first ohmic resistor R₀ to the output of operational amplifier OP.First capacitor C₀ may be connected in parallel to first ohmic resistorR₀ via a second switch S₂. The non-inverting input of operationalamplifier OP is connected via second ohmic resistor R₁ to the output ofoperational amplifier OP. Furthermore, the non-inverting input ofoperational amplifier OP is connected via third ohmic resistor R₂ to thereference potential. The non-inverting input of operational amplifier OPis additionally connectable via a first switch S₁ to the referencepotential. The voltage between the output of operational amplifier OPand the reference potential is output voltage u₀ of astablemultivibrator 15 or signal voltage u_(M) of charge amplifier 25,depending on the selected analysis operating mode. With the aid of bothswitches S₁ and S₂ it is now possible to modify an amplifier circuit, inthis case operational amplifier OP, using further components R₀, R₁, R₂,and C₀ in such a way that the system operates either as an oscillatoraccording to the astable multivibrator shown in FIG. 1 a) or as a chargeamplifier as shown in FIG. 2. First ohmic resistor R₀ is selected insuch a way that it may be used for the operating mode of circuit 30 as acharge amplifier. Dimensioning first ohmic resistor R₀ to establish alower limiting frequency of the charge amplifier and therefore of thesignal processing of the combustion chamber pressure detected via signalvoltage u_(M) is known to those skilled in the art. If it is notpossible to use first ohmic resistor R₀ jointly for both analysisoperating modes, a further switch, not shown in FIG. 4, may be used tochange over between two ohmic resistors instead of first ohmic resistorR₀. The analysis operating mode of circuit 30 is now selected using bothswitches S₁ and S₂: if both switches S₁ and S₂ are closed, circuit 30operates as a charge amplifier as shown in FIG. 2. If both switches S₁and S₂ are open, circuit 30 operates in oscillator operation as theastable multivibrator shown in FIG. 1 a).

Both switches S₁ and S₂ may be implemented as electronic switches, forexample, and are controlled in this example as a function of thecrankshaft angle in such a way that the combustion chamber pressure inthe form of signal voltage u_(M) is analyzed in the range of interest ofthe engine cycle of cylinder 10. In another range of the crankshaftangle, circuit 30 then acts as the astable multivibrator for determiningcapacitance C_(S) of combustion chamber pressure sensor 1.

If the changeover between the two analysis operating modes is controlledin controller 75, the changeover to the particular signal processing ofthe output signal of circuit 30 may thus be performed in controller 75.During the time of oscillator operation, the output signal of circuit 30is then not interpreted as signal voltage u_(M), but rather as outputvoltage u₀ of the astable multivibrator, i.e., not as the characteristicvariable for the combustion chamber pressure. Rather, output voltage u₀is analyzed in this case to determine frequency or period T using acounter in a microprocessor of controller 75, for example, and thereforeto determine capacitance C_(S) of combustion chamber pressure sensor 1.During the time of the charge amplifier operation, the output signal ofcircuit 30 is interpreted as signal voltage u_(M) and therefore as thecharacteristic variable for the combustion chamber pressure. Specificvariables of the combustion may be calculated in controller 75 fromsignal voltage u_(M).

For the oscillator operation of circuit 30 to determine capacitanceC_(S) of combustion chamber pressure sensor 1, ohmic resistors R₀, R₁,and R₂ are suitably dimensioned in such a way that the frequency ofoutput voltage u₀ is in a range in which a sufficient number of periodsof the square wave generated as shown in FIG. 1 b) are generated in thecrankshaft angle range of the second analysis operating mode, and theinternal cycle of the microprocessor of controller 75 allows adequateresolution of the frequency of this square wave. In this way, it isensured that the frequency of the square wave and therefore capacitanceC_(S) of combustion chamber pressure sensor 1 may be determined reliablyduring the second analysis operating mode. With increasing engine speed,the time for the exhaust stroke of cylinder 10 becomes shorter andshorter, so that under certain circumstances the frequency of the squarewaves of output voltage u₀ determined by the dimensioning of ohmicresistors R₀, R₁, and R₂ no longer provides sufficient periods in thecrankshaft angle range for the second analysis operating mode.Therefore, in an alternative embodiment of the present invention, firstohmic resistor R₀, second ohmic resistor R₁, and/or third ohmic resistorR₂ may be replaced by another ohmic resistor in each case, using achangeover circuit, as a function of the engine speed of internalcombustion engine 5 in order to ensure, at different engine speeds, thatthere is a sufficient number of periods of the square wave signal ofoutput voltage u₀ in the crankshaft angle range for the second analysisoperating mode and an adequate resolution of the frequency of thissquare wave is ensured by the internal clock of the microprocessor ofcontroller 75.

In the example shown in FIG. 4, the circuit of the charge amplifiershown in FIG. 2 is combined with the circuit of the astablemultivibrator shown in FIG. 1 a) into single circuit 30 using switchesS₁, S₂, so that it is possible to change over between them. In a similarway, the circuit of the impedance transformer shown in FIG. 3 may becombined with the circuit of the astable multivibrator shown in FIG. 1a) so it is possible to change over between them.

FIG. 5 shows the output signal of operational amplifier OP of circuit 30shown in FIG. 4 plotted against the crankshaft angle in degrees. Duringthe combustion of the air/fuel mixture in combustion chamber 35 ofcylinder 10 in a range between 0° crankshaft angle and approximately550° crankshaft angle and between approximately 700° crankshaft angleand 720° crankshaft angle, the first analysis operating mode is executedand output signal of operational amplifier OP corresponds to signalvoltage u_(M). In this case, first switch S₁ and second switch S₂ areclosed. In the second analysis operating mode between approximately 550°crankshaft angle and approximately 700° crankshaft angle, both switchesS₁ and S₂ are open and switch 30 operates in oscillator operation, i.e.,the voltage u₀ is applied at the output in the form of the approximatelysquare wave, from whose frequency capacitance C_(S) of combustionchamber pressure sensor 1 may be determined in the way described. InFIG. 5, the curve of the combustion chamber pressure is shown for thefirst analysis operating mode in the form of signal voltage u_(M) havinga maximum in the expansion phase of cylinder 10 at approximately 370°crankshaft angle.

Described configurable circuit 30 may be constructed from discreteelectronic components, but also as an integrated circuit. The circuitmay be positioned in the proximity of combustion chamber pressure sensor1, for example, in the same housing, or may be integrated intocontroller 75. Combustion chamber pressure sensor 1 is typically notintegrated into controller 75 in this case.

Switches S₁, S₂ may be electronic components, in the form of transistorsor other semiconductor switches, for example, or may be conventionalelements, such as relays.

The temperature response of piezoelectric elements is known fromcharacteristic curves. If influencing factors, which are not a functionof temperature, on the capacitance of such piezoelectric elements ascombustion chamber pressure sensor 1, which are fixed by materialproperties and geometric dimensions, are constant, capacitance C_(S) ofcombustion chamber pressure sensor 1 and therefore the temperature maybe inferred directly from the oscillation frequency of the square waveshown in FIG. 1 b) in oscillator operation of circuit 30, i.e., thesecond analysis operating mode.

Calibration of the characteristic curve between capacitance C_(S) ofcombustion chamber pressure sensor 1 and the temperature of combustionchamber pressure sensor 1 allows the temperature-independent influencingfactors of capacitance C_(S) of combustion chamber pressure sensor 1 tobe considered. For this purpose, in a further embodiment of the presentinvention, a temperature of sensor element 1, in this case thecombustion chamber pressure sensor, may be measured at a definedoperating point of internal combustion engine 5. At this definedoperating point, the temperature of the combustion chamber pressuresensor is additionally determined in the way described from thefrequency of the square wave of output voltage u₀ in the second analysisoperating mode of circuit 30 via capacitance C_(S) of combustion chamberpressure sensor 1 and the previously described temperature-capacitancecharacteristic curve. This temperature-capacitance characteristic curveis then corrected so that the temperature for assigned capacitance C_(S)of combustion chamber pressure sensor 1 measured in the cited operatingpoint of internal combustion engine 5 is on the temperature-capacitancecharacteristic curve. For this purpose, the characteristic curve must beshifted, while its slope remains the same, until the measuredtemperature is on the characteristic curve associated with capacitanceC_(S) determined at this operating point. For example, directly beforeor after completing the engine start, a temperature may be measuredwhich corresponds as closely as possible to the temperature ofcombustion chamber pressure sensor 1 as the predefined operating pointfor this calibration procedure. If combustion chamber pressure sensor 1is mounted in the cylinder head or on a component in the cylinder head,the coolant temperature is a suitable temperature, whose measured valueapproximately corresponds to the temperature of the combustion chamberpressure sensor directly before or after the engine start, in particularif internal combustion engine 5 has been cooled to the ambienttemperature before the engine start. The coolant temperature measureddirectly after the engine start then also approximately corresponds tothe ambient temperature.

As an alternative to using circuit 30 for executing the two analysisoperating modes, during the first analysis operating mode, the chargeamplifier shown in FIG. 2 or the impedance transformer shown in FIG. 3may be connected to combustion chamber pressure sensor 1 and, during thesecond analysis operating mode, the astable multivibrator shown in FIG.1 a) may be connected to combustion chamber pressure sensor 1, so thatdepending on the analysis operating mode, combustion chamber pressuresensor 1 is connected to a different circuit in each case using aswitch, for example. In this case, to determine capacitance C_(S) ofcombustion chamber pressure sensor 1, combustion chamber pressure sensor1 may also be disconnected during the second analysis operating modefrom the charge amplifier shown in FIG. 2 or from the impedancetransformer shown in FIG. 3 and instead connected to a measuringcircuit. An embodiment of such a measuring circuit is, for example, abridge circuit for directly determining capacitance C_(S) of combustionchamber pressure sensor 1, which is known to those skilled in the art.For analyzing the signal of combustion chamber pressure sensor 1 todetect the measured variable and/or signal voltage u_(M), any otherarbitrary circuits for signal processing known to those skilled in theart may also be used for the connection to combustion chamber pressuresensor 1 during the first analysis operating mode.

In the first analysis operating mode, combustion chamber pressure sensor1 may only be connected to a circuit for analyzing the signal ofcombustion chamber pressure sensor 1 to detect the measured variableusing signal voltage u_(M), for example, and, in the second analysisoperating mode, combustion chamber pressure sensor 1 may only beconnected to a circuit for detecting the characteristic variable ofcombustion chamber pressure sensor 1 which is different from themeasured variable, in this example the capacitance or the temperature ofcombustion chamber pressure sensor 1 using output voltage u₀.

In the exemplary embodiments described above, charge amplifier 25 shownin FIG. 2 and/or impedance transformer 20 shown in FIG. 3 and/or circuit30 having closed switches S₁, S₂ represent first analysis means and theastable multivibrator shown in FIG. 1 a) and/or circuit 30 having openswitches S₁, S₂ shown in FIG. 4 represent second analysis means.

Specific variables of the combustion may be calculated in controller 75in a conventional way from signal voltage u_(M).

A flow chart for an exemplary sequence of the method according to thepresent invention is illustrated in FIG. 7. After the start of theprogram, controller 75 detects instantaneous crankshaft angle KW atprogram point 100 using a crankshaft angle sensor in a way known tothose skilled in the art, for example. Furthermore, controller 75 mayoptionally establish the frequency of setting the second analysisoperating mode as a function of the rate of change of capacitance C_(S)of combustion chamber pressure sensor 1 by predefining a suitable valuen at program point 100, so that the second analysis operating mode isactivated only in every nth cycle of cylinder 10. The rate of change ofcapacitance C_(S) of combustion chamber pressure sensor 1 over time maybe determined by controller 75 from previously determined values forcapacitance C_(S). Furthermore, controller 75 checks at program point100 in which cycle the second analysis operating mode was lastactivated. Subsequently, the program branches to a program point 105.

At program point 105, controller 75 checks whether the currentlydetermined crankshaft angle is in the range between 0° and 550° orbetween 700° and 720°. If so, the program branches to a program point110, otherwise the program branches to a program point 115.

At program point 110, controller 75 causes the closing of both switchesS₁, S₂ and therefore activates the first analysis operating mode.Subsequently, the program is terminated.

At program point 115, controller 75 checks whether, starting from thelast cycle of cylinder 10 in which the second analysis operating modewas activated, the nth cycle of cylinder 10 has been reached again inthe meantime. If so, the program branches to a program point 120,otherwise the program is terminated.

At program point 120, controller 75 causes opening of both switches S₁,S₂ of circuit 30 shown in FIG. 4 and thus activates the second analysisoperating mode. Subsequently, the program is terminated. The programshown in FIG. 7 may be run through repeatedly in this case, inparticular for every new crankshaft angle. In this way, the firstanalysis operating mode and the second analysis operating mode alternateperiodically, the second analysis operating mode not having to occur inevery cycle of cylinder 10 as described. If value n for the frequency ofthe activation of the second analysis operating mode changes, the periodat which the first analysis operating mode and the second analysisoperating mode alternate with one another also changes.

The implementation of the two analysis operating modes using circuit 30shown in FIG. 4 is the basis of the flow chart shown in FIG. 7.

The present invention is not restricted to the use of a combustionchamber pressure sensor for sensor element 1, but rather may be executedin a similar way for any arbitrary sensor elements, in particular forpiezoelectric sensor elements and in particular for pressure sensors.The present invention is also not restricted to the temperature or thecapacitance as the characteristic variable of sensor element 1, butrather is applicable in a corresponding way to any arbitrarycharacteristic variables of sensor element 1. The sole decisive factoris that the signal of sensor element 1, which is produced as a functionof a measured variable and is output by sensor element 1, is analyzed ina first analysis operating mode to detect the measured variable and thesignal of sensor element 1 is analyzed in a second analysis operatingmode to detect a characteristic variable of sensor element 1 which isdifferent from the measured variable. This may be implemented inparticular, as described, by connecting the sensor element to differentcircuits depending on the analysis operating mode, in the first analysisoperating mode the sensor element being connected to a circuit whichanalyzes the signal of sensor element 1 to detect the measured variable.In the second analysis operating mode, the sensor element is connectedto a circuit which analyzes the signal of the sensor element to detectthe characteristic variable of sensor element 1 which is different fromthe measured variable. In this case, only one of the two circuits isalways connected to sensor element 1, depending on the analysisoperating mode.

1. A method for analyzing a sensor element which outputs a signal as afunction of a measured variable, comprising: analyzing the signal of thesensor element in a first analysis operating mode to detect the measuredvariable; and analyzing the signal of the sensor element in a secondanalysis operating mode to detect a characteristic variable of thesensor element which is different from the measured variable, whereinthe sensor element is a combustion chamber pressure sensor for detectinga combustion chamber pressure of an internal combustion engine.
 2. Amethod for analyzing a sensor element which outputs a signal as afunction of a measured variable, comprising: analyzing the signal of thesensor element in a first analysis operating mode to detect the measuredvariable; and analyzing the signal of the sensor element in a secondanalysis operating mode to detect a characteristic variable of thesensor element which is different from the measured variable, wherein achangeover between the first analysis operating mode and the secondanalysis operating mode is performed as a function of at least onecontrolled variable, wherein a crankshaft angle of an internalcombustion engine is the controlled variable.
 3. The method as recitedin claim 1, wherein the second analysis operating mode is set for acylinder of the internal combustion engine during at least one exhauststroke of the cylinder.
 4. A method for analyzing a sensor element whichoutputs a signal as a function of a measured variable, comprising:analyzing the signal of the sensor element in a first analysis operatingmode to detect the measured variable; and analyzing the signal of thesensor element in a second analysis operating mode to detect acharacteristic variable of the sensor element which is different fromthe measured variable, wherein a frequency of setting the secondanalysis operating mode is selected as a function of a rate of change ofthe characteristic variable.
 5. A method for analyzing a sensor elementwhich outputs a signal as a function of a measured variable, comprising:analyzing the signal of the sensor element in a first analysis operatingmode to detect the measured variable; and analyzing the signal of thesensor element in a second analysis operating mode to detect acharacteristic variable of the sensor element which is different fromthe measured variable, wherein one of a temperature or a capacitance isanalyzed as the characteristic variable of the sensor element.
 6. Themethod as recited in claim 5, further comprising: incorporating thesensor element into an oscillator circuit to detect the temperature orcapacitance, the oscillator circuit generating a frequency that is afunction of only the capacitance of the sensor element.
 7. The method asrecited in claim 6, wherein the oscillator circuit is an astablemultivibrator.
 8. A method for analyzing a sensor element which outputsa signal as a function of a measured variable, comprising: analyzing thesignal of the sensor element in a first analysis operating mode todetect the measured variable; analyzing the signal of the sensor elementin a second analysis operating mode to detect a characteristic variableof the sensor element which is different from the measured variable; andincorporating the sensor element into an impedance transformer circuitor into a charge amplifier circuit to detect the measured variable.